APPLIED ELECTRONICS Outcome 1

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Transcript APPLIED ELECTRONICS Outcome 1

MUSSELBURGH GRAMMAR SCHOOL
APPLIED ELECTRONICS
Outcome 1
Gary Plimer 2004
APPLIED ELECTRONICS Outcome 1
Outcome 1 - Design and construct electronic systems to meet given
specifications
When you have completed this unit you should be able to:
 State and carry out calculations using the current gain and
voltage gain equations.
 Carry out calculations involving bipolar transistor switching
circuits.
 Carry out calculations involving MOSFET transistor circuits.
 Identify and describe the uses of transistors in “push-pull”
circuits.
 Carry out calculations involving Darlington pair circuits.
 Design transistor circuits for a given purpose.
APPLIED ELECTRONICS Outcome 1
Before you start this unit you should have a basic understanding of:
 Input and Output transducers
 Voltage divider circuits
 Ohm’s Law - relationship between V and I in a d.c. circuit
 Kirchoff’s laws for current and voltage
 The operational characteristics of various electronic components
 Use of breadboards
 Use of circuit test equipment: multimeter and oscilloscope
APPLIED ELECTRONICS Outcome 1
Any electronic system can be broken down into three distinct parts
INPUT
PROCESS
OUTPUT
We are going to start by looking at
INPUT TRANSDUCERS
INPUT transducers convert a change in physical conditions (e.g. temperature)
into a change in an electrical property (e.g. voltage) which can then be
processed electronically to produce either a direct measurement of the physical
condition (temperature in oC) or to allow something to happen at a
predetermined level (e.g. switching ON the central heating at 20 °C).
APPLIED ELECTRONICS Outcome 1
Changes in the resistance of an input transducer must be converted to
changes in voltage before the signal can be processed. This is normally
done by using a voltage divider circuit.
Vcc
Voltage divider circuits work on the basic
electrical principle that if two resistors are
connected in series across a supply, the
voltage load across each of the resistors will
be proportional to the value of the resistors.
R
-t
0V
Vout
(Signal)
R2
Signal Voltage 
 VCC
Rtotal
APPLIED ELECTRONICS Outcome 1
Common Input Transducers
Physical condition to be
monitored
Input Transducer
Electrical property that
changes
Temperature
Thermistor
Thermocouple
Platinum Film
Resistance
Voltage
Resistance
Light
LDR
Selenium Cell
Photo Diode
Resistance
Voltage/current
Current/Resistance
Distance
Slide Potentiometer
Variable Transformer
Variable Capacitor
Resistance
Inductance
Capacitance
Force
Strain Gauge
Resistance
Angle
Rotary Potentiometer
Resistance
APPLIED ELECTRONICS Outcome 1
PUPIL ASSIGNMENT 1
Calculate the signal voltages produced by the following voltage divider circuits:
5V
9V
12V
10k
-t
5k
2k
Vout
(Signal)
0V
Vout
(Signal)
5k
15k
Vout
(Signal)
4k7
0V
5V
3k3
Vout
(Signal)
10k
2V
0V
Vout
(Signal)
0V
10V
Vout
(Signal)
2k
0V
12V
8k2
3k3
4k7
0V
5V
0V
3k
10V
1.5V
0V
APPLIED ELECTRONICS Outcome 1
AMPLIFICATION and BIPOLAR JUNCTION TRANSISTORS
Input transducers rarely produce sufficient voltage to operate
output transducers, (motors, bulbs, etc.) directly.
To overcome this problem, we need to AMPLIFY their output
voltage or current.
Amplifying devices are said to be active components as opposed
to non-amplifying components such as resistors, capacitors etc.
which are known as passive components.
The extra energy required to operate the active component
comes from an external power source such as a battery,
transformer, etc.
APPLIED ELECTRONICS Outcome 1
AMPLIFICATION and BIPOLAR TRANSISTORS
The most common active device in an electronic system is the Bipolar Junction
Transistor (or simply transistor for short). Two types are available, NPN or PNP.
Collector
Collector
The transistor has to be
Base
connected into circuits
correctly. The arrow head on
the emitter indicates the
direction of "conventional"
Em itter
current flow (positive-tonegative).
npn Type
NPN transistors operate when the base is made
Positive
PNP transistors operate when the base is made
Negative
Base
Em itter
pnp Type
APPLIED ELECTRONICS Outcome 1
TRANSISTOR NOTATION
Subscripts are normally used to indicate specific Voltages and Currents
associated with transistor circuits,
Ic
Ib
Ie
VCC
Vb
Ve
Vce
Vbe
VL
-
Collector current
Base current
Emitter current
Voltage of supply (relative to ground line)
Voltage at the base junction (relative to ground line)
Voltage at the emitter junction (relative to ground line)
Voltage between the collector and emitter junction
Voltage between the base and emitter junction
Voltage over the load resistor
Rb
Vcc
RL
VL
Ic
Ib
Vce
Vbe
Vin
Vc
Ie
Vb
Re
Ve
Ve
0V
APPLIED ELECTRONICS Outcome 1
Common Emitter Mode


The transistor can be used in
different modes, the most
common of which is the
common emitter mode.
(So called because the emitter
is common to both input and
output signals.)
In the common emitter mode, a
small current flowing between
the base and emitter junction
will allow a large current to flow
between the collector and
emitter.
b
OUTPUT
e
INPUT
COMMON LINE
c
b
b
c
e
APPLIED ELECTRONICS Outcome 1
Common Emitter Mode
c
It can be seen that :
b
Ie = Ib + Ic
b
c
Since Ib is usually much smaller than Ic,
it follows that Ie is approximately = Ic
e
APPLIED ELECTRONICS Outcome 1
Common Emitter Mode
Current Gain

The bipolar transistor is a current-controlled amplifying
device

The current gain (or amplification) of the transistor is
defined as the ratio of collector / base currents
current
Collector current
gain 
Base current
Ic
AI 
Ib
APPLIED ELECTRONICS Outcome 1
Common Emitter Mode

Current Gain
The accepted symbol for transistor current gain in dc mode is,
hFE

The maximum allowable currents will depend on the make of
transistor used. These limits can be obtained from
manufacturers' data sheets.

Forcing the transistor to carry currents greater than these
maxima will cause the transistor to overheat and may damage it.

If the transistor is used to amplify a.c. signals then the gain is
defined as,
I c
h fe 
I b
APPLIED ELECTRONICS Outcome 1
Common Emitter Mode
Current Gain
Pupil Assignment 2
1. Calculate the gain of a transistor if the collector current is
measured to be 10 mA when the base current is 0.25 mA.
2. Calculate the collector current through a transistor if the
base current is 0.3 mA and hFE for the transistor is 250.
3. What collector current would be measured in a BC107
transistor if the base current is 0.2 mA and hFE is
100?
4. In questions 2 & 3, are the transistors ac or dc ? Explain
why.
APPLIED ELECTRONICS Outcome 1
TRANSISTOR SWITCHING CIRCUITS


In order to generate a
current in the base of
the transistor, a
voltage must be applied
between the base emitter junction (Vbe).
It is found that no (or at
least negligible) current
flows in the base circuit
unless Vbe is above 0.6
Volts.
c
b
b
c
e
APPLIED ELECTRONICS Outcome 1
TRANSISTOR SWITCHING CIRCUITS


Increasing the base - emitter voltage
further, increases the base current,
producing a proportional increase in the
collector current.
c
b
When the base - emitter voltage
reaches about 0.7 V, the resistance
between the base emitter junction starts
to change such that the base - emitter
voltage remains at about 0.7 V.
b
c
e

At this point the transistor is said to be saturated. Increasing the
base current further has no effect on the collector current. The
transistor is fully ON.

It can be assumed that if the transistor is turned ON, Vbe = 0.7 V
APPLIED ELECTRONICS Outcome 1
Pupil Assignment 3

For each of the circuits shown, calculate Vbe and state if the
transistor is ON or OFF.
APPLIED ELECTRONICS Outcome 1
TRANSDUCER DRIVER CIRCUITS

Output transducers can require
large currents to operate them.
Vcc

Currents derived from input
transducers, either directly, or
from using a voltage divider
circuit tend to be small.

A transistor circuit can be used
to drive the output transducer.

A small current into the base of
the transistor will cause a large
current to flow in the collector/
emitter circuit into which the
output transducer is placed.
OUTPUT
TRANSDUCER
Ic
Ib
0V
APPLIED ELECTRONICS Outcome 1
Vcc
TRANSDUCER DRIVER CIRCUITS



The base current is derived from
applying a voltage to the base of
the transistor.
If the voltage between the base emitter junction (Vbe) is less than
0.6 V, the transistor will not
operate, no current will flow in
the emitter/collector circuit and
the output transducer will be
OFF.

If Vbe is 0.7 V (or forced above
0.7 V), the transistor will operate,
a large current will flow in the
emitter/collector circuit and the
transducer will switch ON.
OUTPUT
TRANSDUCER
Ic
Ib
0V
If Vbe lies between 0.6
and 0.7, the transistor acts
in an analogue manner
and this may result in the
output transducer
hovering around an on
and off state
APPLIED ELECTRONICS Outcome 1
Vcc
Worked Example

If the transistor is FULLY ON,
calculate the collector
current and Vce , if hFE
=200 and VCC = 9 Volts
150k
470R
Step 1
The voltage between the base and emitter junction is
always about 0.7 V
Since the emitter is connected to the ground line (0V),
Vb= 0.7 V
Step 2
The voltage dropped over the base resistor can then be
calculated.
Voltage drop = VCC - Vb = 9 - 0.7 = 8.3V
0V
APPLIED ELECTRONICS Outcome 1
Vcc
Worked Example continued
Step 3
The base current is calculated using
Ohm's law
Ib 
Vdropped
Rb
150k
470R
8.3

 00.0553
.00553mA
mA
150k
0V
Step 4
Ic is calculated knowing hFE
Ic = hFE x Ib = 200 x 0.0553 = 11.06 mA
APPLIED ELECTRONICS Outcome 1
Vcc
Worked Example continued
150k
470R
Step 5
VL is calculated using Ohm's law
VL = Ic x RL = 11.06 mA x 470 = 5.2 V
Step 6
Vce is calculated
Vce = Vcc - VL = 9 - 5.2 = 3.8 V
0V
APPLIED ELECTRONICS Outcome 1
Pupil Assignment 4
A 6 V, 60 mA bulb is connected to the collector of a BFY50
transistor as shown below.
9V
If the gain of the transistor is
30, determine the size of the
base resistor Rb required to
ensure that the bulb
operates at its normal
brightness.
Rb
6V, 60mA
BFY50
0V
APPLIED ELECTRONICS Outcome 1
VOLTAGE AMPLIFICATION




Although the transistor is a current amplifier, it can easily be
modified to amplify voltage by the inclusion of a load resistor, RL
in the collector and/or emitter line.
If we apply a voltage Vin to the base
of the transistor, the base current Ib
will flow.
This will causes a proportional
increase (depending on the gain) of
the collector current Ic.
Since the current through the load
resistor (Ic) has increased, the
voltage over RL has increased (VL =
IcRL) and hence Vout has decreased.
(Vout = VCC - VL)
Vcc
VL
RL
Ic
Ib
BFY50
Vout
Vin
0V
APPLIED ELECTRONICS Outcome 1
VOLTAGE AMPLIFICATION (continued)
The Voltage gain of any amplifier is defined as
Vcc
Voltage
voltage output
gain 
voltage input
VL
RL
Ic
Ib
Vo
AV 
Vi
BFY50
Vout
Vin
0V
APPLIED ELECTRONICS Outcome 1
Vcc
WORKED EXAMPLE
1k

Calculate the voltage gain of
this circuit if,
Vin =1.7 Volt,
hFE = 100
and VCC = 6V
Vout
Vin
2k
0V
APPLIED ELECTRONICS Outcome 1
WORKED EXAMPLE
Vcc
Step 1
The voltage between the base and
emitter junction (Vbe) is always about
0.7 V hence:
RL
VL
Ve = Vin - 0.7 = 1.0 V
Vcc
Step 2
The current through Re is calculated
using Ohm's law
Vin
Ve
V
10
.
Ie  e 
 0.5mA
Re 2 k
Vout
Vbe
Re
0V
APPLIED ELECTRONICS Outcome 1
WORKED EXAMPLE
Vcc
Step 3
For this value of hFE, Ib will be
small compared to Ic (one
hundredth of the value), hence,
RL
VL
Ic = Ie
Vcc
Vout
Vbe
Step 4
The voltage over the load resistor (RL) is
calculated using Ohm's law
VL = Ic x RL = 0.5 mA x 1k = 0.5 V
Vin
Ve
Re
0V
APPLIED ELECTRONICS Outcome 1
Vcc
WORKED EXAMPLE
1k
0.5V
Step 5
The output voltage can now be
calculated from
Vout
0.5mA
= VCC - VL
= 6 - 0.5 = 5.5 V
6.0V
0.7V
5.5V
1.7V
Step 6
The voltage gain is
therefore
2K
1.0V
2k2
0.5mA
AV 
Vo
Vi
5.5/1.7 = 3.2
0V
APPLIED ELECTRONICS Outcome 1
Vcc
Pupil Assignment 5
2k
A transistor of very high current gain is
connected to a 9 Volt supply as shown.
Determine the output voltage and
the voltage gain when an input of 3
Volts is applied.
Vout
Vin
4k7
0V
APPLIED ELECTRONICS Outcome 1
Practical Considerations

Care must be taken to
ensure that the maximum
base current of the
transistor is not exceeded.

When connecting the base
of a transistor directly to a
source, a base protection
resistor should be included.
This will limit the maximum
current into the base.

Most data sheets will quote
the maximum collector
current and hFE and so the
maximum allowable base
current can be calculated.
Vin
0V
APPLIED ELECTRONICS Outcome 1
Practical Considerations
CURRENT FLOWING
FROM Vcc INTO BASE
If the transistor is to be connected to a
potential divider circuit then the
maximum possible current into the base
will depend on R1
The maximum possible current
through R1 (and hence into the base)
would be =
VCC
R1
hence if R1 is large, the base current
will be small and therefore no
damage should occur.
Vcc
R1
RL
R2
Re
0V
APPLIED ELECTRONICS Outcome 1
Practical Considerations
If R1 is small (or has the capability of
going small e.g. using a variable resistor
as R1), a protection resistor must be
included in the base.
Vcc
RL
R1
Rb
If R1 = 0, the maximum possible current
into the base =
VCC
Rb
hence Rb can be calculated if VCC and the
maximum allowable base current is known.
R2
Re
0V
APPLIED ELECTRONICS Outcome 1
Pupil Assignment 6
9V
Assume Ic(max) for the
transistor shown is 100 mA and
hFE is 200.
Rb
Vin = 5V
0V
Calculate:

The maximum allowable base current.
 The size of protection base resistor
required
(remembering Vbe =
0.7V, and R = V/I)
APPLIED ELECTRONICS Outcome 1
CIRCUIT SIMULATION

It is possible to use circuit simulation software such as ‘Crocodile
Clips’ to investigate electric and electronic circuits.

Circuit simulation is widely used in industry as a means of
investigating complex and costly circuits as well as basic circuits.

Circuit simulators make the modelling and testing of complex circuits
very simple.

The simulators make use of libraries of standard components along
with common test equipment such as voltmeters, ammeters and
oscilloscopes.
Question:
What do you think the main advantage of
simulation of circuits is?
APPLIED ELECTRONICS Outcome 1
CIRCUIT SIMULATION (Base Protection)

Using the simulation
software, construct the
circuit shown, using a 5 V
supply.

Switch on and see what
happens.

Now insert a 10k base
protection resistor and see
what happens when you
switch on now.

Use the simulation to
determine the smallest value
of resistor required to
protect this transistor.
5V
5V
APPLIED ELECTRONICS Outcome 1
CIRCUIT SIMULATION (Base Protection)
Construct the circuit shown.
100R


See what happens
when you reduce the
size of the variable
resistor.
Now re-design the
circuit to include a
base protection
resistor.
5V
10k
APPLIED ELECTRONICS Outcome 1
Pupil assignment 7
An NTC thermistor is used in the circuit shown below to indicate if the
temperature falls too low. When the bulb is on the current through it is 60 mA.
6V
10k
RESISTANCE()
2000
GRAPH OF R/T
FOR
NTC THERMISTER
1500
1000
-t
500
TEMP(C)
0V
10
20
30
40
APPLIED ELECTRONICS Outcome 1
Pupil assignment 7







If hFE for the transistor is 500, determine the base current required
to switch on the bulb.
What voltage is required at the base of the transistor to ensure that
the bulb indicator switches ON?
Calculate the voltage dropped over, and hence the current through
the 10 k resistor.
Calculate the current through the thermistor and the resistance of
the thermistor when the bulb is ON?
Using the information on the graph, determine at what temperature
the bulb would come ON.
How could the circuit be altered so that the bulb would come on at a
different temperature?
How could the circuit be altered so that the bulb would come when
the temperature is too high?
APPLIED ELECTRONICS Outcome 1
Pupil assignment 8
For each of the circuits, calculate the base current, the emitter voltage and current
10V
1k
12V
4k
hFE = 200
2k
hFE = 50
100R
2k
0V
1k
0V
APPLIED ELECTRONICS Outcome 1
Pupil assignment 8
For each of the circuits, calculate the base current, the emitter voltage and current
9V
5V
2k
3k
hFE = 100
hFE = 200
2k
220R
6k
0V
470R
0V
APPLIED ELECTRONICS Outcome 1


In order to obtain higher gains, more
than one transistor can be used, the
output from each transistor being
amplified by the next (known as
cascading).
RL
Increasing the gain of the circuit
means:
The switching action of the circuit is
more immediate;
A very small base current is required
in switching;
The input resistance is very high.

The Darlington Pair
A popular way of cascading two
transistors is to use a Darlington pair
(Named after the person that first
designed the circuit)
Tr1
Tr2
APPLIED ELECTRONICS Outcome 1
The Darlington Pair


The current gain of the "pair" is
equal to the product of the two
individual hFE's.
If two transistors, each of gain 50 are
used, the overall gain of the pair will
be 50 x 50 = 2500
RL
Tr1
AI  hFE 1  hFE 2
Because of the popularity of this circuit design, it is possible to buy a
single device already containing two transistors
Tr2
APPLIED ELECTRONICS Outcome 1
The Darlington Pair



In a Darlington pair, both
transistors have to be switched
on since the collector-emitter
current of Tr1 provides the
base current for Tr2.
In order to switch on the pair,
each base-emitter voltage
would have to be 0.7V
The base-emitter voltage
required to switch on the pair
would therefore have to be
1.4V.
0.7V
1.4V
0.7V
0V
APPLIED ELECTRONICS Outcome 1
Worked Example
For the Darlington pair shown,
calculate:

The gain of the pair;

The emitter current;

The base current
hFE1 = 200
hFE2 = 50
8V
27R
0V
APPLIED ELECTRONICS Outcome 1
Worked Example
Step 1
The overall gain = product
of the individual gains
hFE1 = 200
AI  hFE 1  hFE 2  200  50  10000
Step 2
The voltage over the load resistor
must be the input voltage to the
base minus the base-emitter
voltage required to switch on the
pair
VL = Vin - Vbe = 8 - 1.4 = 6.6 V
hFE2 = 50
8V
27R
0V
`
Worked Example
Step 3
The emitter current in the load
resistor can be obtained from
Ohm’s law
hFE1 = 200
V L 6.6
Ie 

 0.244 A
R L 27
Step 4
hFE2 = 50
8V
Since the gain is very high, Ic = Ie
and the gain for any transistor
circuit = Ic/Ib
hence knowing Ic and AI, Ib can be
calculated
Ai 
Ic
I
0.244
 Ib  c 
 24.4  10 6 A
Ib
Ai 10000
27R
0V
APPLIED ELECTRONICS Outcome 1
Pupil Assignment 9
For the circuit shown, the gain of
Tr1 is 150, the gain of Tr2 is 30.
RL
Calculate:

The overall gain of the
Darlington pair;

The base current required
to give a current of 100
mA through the load
resistor.
Tr1
Tr2
APPLIED ELECTRONICS Outcome 1
MOSFETS

Although the base current in a transistor is usually small (< 0.1 mA),
some input devices (e.g. a crystal microphone) have very small
output currents. In many cases, this may not be enough to operate
a bipolar transistor.

In order to overcome this, a Field Effect Transistor (FET) can be
used.
COLLECTOR
DRAIN
BASE
GATE
EMITTER
BIPOLAR TRANSISTOR
SOURCE
FIELD EFFECT
TRANSISTOR
APPLIED ELECTRONICS Outcome 1
MOSFETS


Applying a voltage to the Gate connection allows
current to flow between the Drain and Source
connections.
DRAIN
This is a Voltage operated device.
GATE

It has a very high input resistance (unlike the
transistor) and therefore requires very little current
to operate it (typically 10-12 mA).

Since it operates using very little current, it is easy
to destroy a FET just by the static electricity built up
in your body.

FET’s also have the advantage that they can be
designed to drive large currents, they are therefore
often used in transducer driver circuits
SOURCE
FIELD EFFECT
TRANSISTOR
APPLIED ELECTRONICS Outcome 1
D
MOSFETS

Two different types of FET’s are
available:
JFET (Junction Field Effect Transistor)
G
G
S
N-CHANNEL JFET
D
G
G
All FET’s can be N-channel or Pchannel.
S
P-CHANNEL JFET
D
MOSFET (Metal Oxide Semiconductor
Field Effect Transistor)

D
S
N-CHANNEL
ENHANCEMENT
MOSFET
S
P-CHANNEL
ENHANCEMENT
MOSFET
Enhancement-type MOSFET's can be used in a similar way to bipolar transistors.
N-channel enhancement MOSFET’s allow a current to flow between Drain and
Source when the Gate is made Positive (similar to an NPN transistor).
P-channel enhancement MOSFET’s allow a current to flow between Drain and
Source when the Gate is made Positive (similar to an PNP transistor
APPLIED ELECTRONICS Outcome 1
MOSFETS

The simplicity in construction of the MOSFET means that it occupies very
little space.

Because of its small size, many thousands of MOSFET’s can easily be
incorporated into a single integrated circuit.

The high input resistance means extremely low power consumption
compared with bipolar transistors.

All these factors mean that MOS technology is widely used within the
electronics industry today.
APPLIED ELECTRONICS Outcome 1
MOSFETS



ID
Like a bipolar transistor, if the Gate
voltage is below a certain level (the
threshold value, VT), no current will
flow between the Drain and Source
(the MOSFET will be switched off).
If the Gate voltage is above
VT, the MOSFET will start to
switch on.
Increasing the Gate voltage
will increase ID
DRAIN
VDS
GATE
SOURCE
VGS
0V
APPLIED ELECTRONICS Outcome 1
MOSFETS


ID
For a given value of VGS
(above VT), increasing VGS
increases the current until
saturation occurs.
Any further increase will cause
no further increase in ID. The
MOSFET is fully ON and can
therefore be used as a switch.
DRAIN
VDS
GATE
SOURCE
VGS
0V
Saturation occurs when VDS = VGS - VT.
APPLIED ELECTRONICS Outcome 1
5V
Worked Example
100R
The threshold gate voltage for the MOSFET
shown is 2 V. Calculate the gate voltage
required to ensure that a saturation current
of 10 mA flows through the load resistor.
Step 1
The Drain - Source channel acts as a
series resistor with the 100R, since the
current is the same in a series circuit,
the voltage over the 100R can be
calculated.
Using Ohm’s law
V = IR = 10 mA x 100 = 1 Volt
0V
APPLIED ELECTRONICS Outcome 1
Worked Example
Step 2
Using Kirchoff’s 2nd law,
the voltage over the channel + the
voltage over the load resistor = supply
voltage
5V
100R 1V
VDS
hence VDS = 5 - 1 = 4 Volts
0V
Step 3
For saturation to occur,
VDS = VGS-VT
VGS = VDS + VT
VGS = 4 + 2 = 6 Volts.
APPLIED ELECTRONICS Outcome 1
MOSFETS
MOSFET’s can be designed to handle very high drain currents, this means
that they can be used to drive high current output transducers drivers without
the need for relay switching circuits (unlike the bipolar transistor).
Vcc

The load resistor could be any
output transducer, bulb, motor,
relay etc.

Since MOSFET’s are particularly
sensitive to high voltages, care
must be taken to include a reverse
biased diode over transducers that
may cause a back emf when
switched off.
RL
Vin
0V
APPLIED ELECTRONICS Outcome 1
Possible application of a mosfet
Vcc
A variable resistor can be
used in a voltage divider
circuit and adjusted to ensure
that the input voltage to the
gate = VT
RL
The load resistor could be a
bulb, motor, relay coil, etc.
0V
APPLIED ELECTRONICS Outcome 1
The Push-Pull Amplifier



NPN bipolar transistors and
n-type enhancement
MOSFETs operate when the
base or gate is made
positive with respect to the
zero volt line.
PNP and p-type MOSFETs
operate off negative signals.
A push-pull amplifier
consists of one of each type
of bipolar transistor (or
MOSFET) connected in
series with a + and - supply
rail.
+
NPN
Vin
OV
PNP
RL
_
APPLIED ELECTRONICS Outcome 1
The Push-Pull Amplifier
+


If Vin is Positive with
respect to 0V, the NPN
transistor will switch on,
current will flow from the
+ supply line through the
collector-emitter junction,
through the load resistor
down to the 0Volt line
NPN
Vin
If Vin is Negative with respect to 0V, the
PNP transistor will switch on,
current will flow from the 0Volt line
through load resistor, through the
emitter- collector junction, to the supply line.
OV
PNP
RL
_
APPLIED ELECTRONICS Outcome 1
The Push-Pull Amplifier
+
NPN


The direction of current
flow through the load
resistor will therefore
depend on whether the
input voltage is positive or
negative.
If the load resistor is
replaced by a motor, the
direction of rotation of the
motor can be altered
dependent on the input
voltage, Vin.
Vin
OV
PNP
RL
_
APPLIED ELECTRONICS Outcome 1
Circuit simulation
Using Crocodile Clips construct the following circuit.
Investigate what
happens when
the potentiometer
slider is altered
5V
1k
10k
5V
APPLIED ELECTRONICS Outcome 1
Circuit simulation
Using Crocodile Clips construct the following circuit.
Set the resistance
of each LDR to
the same value.
+5V
1k
Set the variable
resistor to its
middle position.
1k
0V
-5V
Alter the value of
one LDR and
observe the
motor.
Alter the value of the other
LDR and observe the motor.
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1995, Paper 1, question 2
12V
The following electronic system
is set up for a test with various
ammeters and voltmeters
connected as shown.
1k
In the condition shown, the
transistor is fully saturated with
a base current of 5mA.
Write down the readings which
you would expect to see on
each of the four voltmeters (V1
- V4) and the two ammeters
(A1 - A2).
300R
0V
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1994, Paper 1, question 1
A designer is asked to construct
an electronic circuit which will
energise a relay at a set light
level. Having investigated the
characteristics of the light
transducer, she finds that the
resistance of the transducer at
“switch on” level is 2.1 M . The
proposed design is shown
opposite.
The transistor saturates when
Vbe = 0.6V.
Vcc
+9V
RELAY
(RESISTANCE 180R)
200k
R
0V
Determine, assuming the transistor is in a
fully saturated condition:
(a) The value of the unknown
resistor R required to make the
transistor operate correctly;
(b)
The power dissipated in the relay
coil.
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1993, Paper 1, question 2
The control circuit for a cooling fan is based on a thermistor.
The graph shows the operating characteristics of the thermistor and the
proposed circuit diagram is also shown.
+6V
300
RELAY
RESISTANCE (k)
-t
200
FAN MOTOR
100
10k
0V
100
200
TEMPERATURE (C)
300
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1993, Paper 1, question 2 Continued
(a)
The motor should switch on when Vbe = 0.6V. For this condition, calculate the
value of Rt.
From the graph, determine the temperature at which the fan should switch on.
(b)
When the circuit is built and tested, it is found that the relay does not operate at
the switch - on temperature.
Suggest one reason why the transistor fails to operate the relay.
Redraw the circuit diagram to show how a Darlington pair could be used to
overcome this problem.
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1998, Paper 2, question 4 (c)
(amended)
An instant electric shower is
designed to deliver water at a
fixed temperature from a cold
water supply.
An additional safety feature is to
be added which will switch off
the power to the shower if the
water temperature produced by
the heating element becomes
dangerously high (greater than
50 oC).
The relay requires an operating
current of 250 mA. The
resistance of the thermistor at
50oC is 1 k.
+12V
-t
RELAY OPERATING
CURRENT 250mA
12k
hFE = 100
Ib
R
hFE 100
0V
APPLIED ELECTRONICS Outcome 1
SEB & SQA Past Paper exam Questions
1998, Paper 2, question 4 (c)
Continued,
Name the transistor configuration
used in this circuit.
State one advantage of using this
configuration.
For the relay to operate:
calculate the base current, Ib;
calculate the potential difference
across the 12kresistor; determine
the voltage across the fixed
resistor R; calculate the value of
R.
+12V
-t
RELAY OPERATING
CURRENT 250mA
12k
hFE = 100
Ib
R
hFE 100
0V